Sponge titanium, method for producing sponge titanium, and method for producing titanium ingot or titanium alloy ingot

ABSTRACT

Provided is sponge titanium produced by the Kroll method, in which the total of a chlorine content and a magnesium content is 350 ppm by mass or lower, and a filling density is 1.65 g/cm3 to 1.95 g/cm3. The present invention can provide sponge titanium for large ingot production that is difficult to cause problems due to chloride inclusion at the time of melting production of the large ingot by a melting method not associated with compression molding and has easy component control and also provide a method for industrially efficiently producing the sponge titanium.

TECHNICAL FIELD

The present invention relates to sponge titanium produced by the Kroll method in which titanium tetrachloride is reduced by metal magnesium to produce sponge titanium, a method for producing the sponge titanium, and a method for producing a titanium ingot or a titanium alloy ingot.

BACKGROUND ART

The sponge titanium is industrially produced by the Kroll method. The process in the industrial production of the sponge titanium by the Kroll method is roughly classified into four steps of a chlorination-distillation step, a reduction-separation step, a crushing step, and an electrolysis step.

The reduction-separation step, one of these steps, is composed of a reduction step and a vacuum separation step. In the reduction step, reduction reaction occurs by adding titanium tetrachloride dropwise onto melted-state metal magnesium in a stainless steel or steel reaction container to generate the sponge titanium and magnesium chloride, which is a by-product. Subsequently, in the vacuum separation step, a sponge titanium block from which residual magnesium chloride and metal magnesium are eliminated is produced by vacuuming the sponge titanium generated in the reduction step in high temperature and reduced pressure (Non-Patent Literature 1).

Thus, produced sponge titanium block is cut or crushed stepwise in the subsequent crushing step and finally the sponge titanium block becomes sponge titanium having a size in millimeter order or centimeter order. The sponge titanium is melted as a main raw material in the following melting step singly or in a combination of auxiliary materials to form titanium ingots or titanium alloy ingots. The term “auxiliary raw materials” here refers to titanium scrap such as turnings of titanium products, plates, and blocks, and in addition, additive materials such as grains, plates, and blocks of alloy elements.

As a method for melting titanium, a consumption electrode type arc melting method, an electron beam melting method, a plasma melting method, a vacuum induction melting method, and an inactivation induction melting method are generally used. In any of the methods, it has been known that the chlorine content of the sponge titanium is an important factor dominating melting stability and productivity.

For example, in the electron beam melting method, the melting is carried out under vacuum. Sponge titanium having a high chlorine content generates large amount of splash associated with volatilization of chlorides and thus a problem of worsening a yield of melting (Non-Patent Literature 2) and a problem in that raw material insertion is improbable due to attaching or depositing the splash on a raw material feed opening (Patent Literature 1) may arise. In addition to this problem, a problem of inhibiting electron beam generation due to volatilized chlorides (Non-Patent Literature 3), a problem of corroding melting facilities due to generated chloride gas, and other problems may also arise. Therefore, as the chlorine content becomes lower, the melting stability is more improved, which is preferable. In the plasma melting method, the vacuum induction melting method, and the inactivation induction melting method, various similar problems associated with the volatilization of chlorides arise and thus the chlorine content in the sponge titanium is preferably as low as possible.

Here, referring to the general chlorine content in the sponge titanium, a chlorine content of about 700 ppm and a magnesium content of about 250 ppm are general values. Among the contents for which the inventors of the present invention have searched, the lowest contents are a chlorine content of 230 ppm and a magnesium content of 140 ppm (Non-Patent Literature 2).

In Non-Patent Literature 4, the chlorine content in the sponge titanium and the residual mechanism of the chlorine have been described in detail. It has been reported that the chlorine distribution in the sponge titanium block is about 1,000 ppm to about 1,500 ppm at the upper part of the block and about 400 ppm to about 600 ppm at the lower part of the block, and the existing states of the chlorine are three states of Type 1: magnesium chloride existing in fine pores in primary particles of titanium, Type 2: magnesium chloride existing between the primary particles of titanium, and Type 3: titanium dichloride attached to the surfaces of the primary particles of titanium, and Type 2 is the main existing state. Here, the term “primary particles of titanium” refers to the particles of titanium having a size of serval tens of micrometers and constituting the sponge titanium. The sponge titanium is a porous product formed by sintering the primary particles.

In Type 1, magnesium chloride exists in the primary particles of titanium in an extremely fine dispersion state. In Type 2, magnesium chloride exists in the gaps between the primary particles of titanium. In Type 3, titanium dichloride exists on the surfaces of the primary particles of titanium.

In Non-Patent Literature 4, as an important factor of dominating the chlorine content, the feed rate of titanium tetrachloride and the existing amount of magnesium in the reduction step have been exemplified. It has been reported that as the feed rate of titanium tetrachloride becomes slower and the existing amount of magnesium becomes larger, the chlorine content becomes lower. At the lower part of the sponge titanium block generated in a state of a slower feed rate of titanium tetrachloride of 1.4 (L/m²/minute) (2.4 (kg/m²/minute)) and sufficient existence of magnesium, the chlorine content is successfully reduced to slightly lower than 400 ppm.

Here, the quality of the titanium ingot is an important factor in the melting production of the ingot. The casting surface and component of the ingot are particularly important.

In the case where the chlorine content in the sponge titanium is high, troubles in which electron beams are out of alignment or electron gun temporarily stops due to the chloride in the melting step frequently occur. Consequently, the outer peripheral part of the melted titanium cannot be irradiated with electron beams. This causes poor casting surface.

With regard to the casting surface, an example of the relation between beam output and reduction rate at the time of electron beam melting and the casting surface has been reported (Non-Patent Literature 2).

With regard to the component, an example of examination of the component distribution of an ingot produced by melting by the electron beam melting method has been recorded (Non-Patent Literature 5). What is particularly important in component control is an oxygen concentration and an iron concentration. For industrial pure titanium, these concentrations are required to be controlled in a narrow range of about 250 ppm.

CITATION LIST Non-Patent Literatures

-   Non-Patent Literature 1: Shigen to Sozai (Journal of the Mining and     Materials Processing Institute of Japan) Vol. 109, Page 1157-1163     (1993) -   Non-Patent Literature 2: ISIJ Vol. 31 (1991) No. 2, Page 906-914 -   Non-Patent Literature 3: Tetsu-to-Hagane (Journal of Iron and Steel)     Vol. 74 (1988), No, 2, Page 215-223 -   Non-Patent Literature 4: Shigen-to-Sozai (Journal of the Mining and     Materials Processing Institute of Japan) Vol. 112, Page 251-256     (1996) -   Non-Patent Literature 5: Zairyou-to-Purosesu (Current Advances in     Materials and Processes) Vol. 1, (1988), No. 5, Page 1335

Patent Literature

-   Patent Literature 1: Japanese Examined Utility Model Application     Publication No. H6-23918

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

However, in the melting step, a chlorine content in the sponge titanium of slightly lower than 400 ppm described in Non-Patent Literature 5 is not a sufficient chlorine reduction amount that solves the problems due to chloride inclusion and thus the problems due to the chloride inclusion still remain. In addition, in the method described in Non-Patent Literature 5, the feed rate of titanium tetrachloride is extremely slow and thus, for industrialization, a problem of excessively low productivity of sponge titanium also arises.

Therefore, sponge titanium not causing the problems due to chloride inclusion and having a low chlorine content in the melting methods not associated with compression molding such as the electron beam melting method, the plasma melting method, the vacuum induction melting method, and the inactivation induction melting method is required.

In the method described in Non-Patent Literature 2, the casting surface can be improved in a small production level. However, at the time of melting production of large ingots having a size of several tons or larger, the poor casting surface is frequently generated even if the beam output and the reduction rate are appropriately controlled. Consequently, improvement is required.

At the time of melting production of large ingots having a size of several tons or larger, the component cannot be controlled frequently within the predetermined range. Consequently, improvement is required.

Therefore, an object of the present invention is to provide sponge titanium for large ingot production that is difficult to cause the problems due to chloride inclusion at the time of melting production of the large ingot by the melting method not associated with the compression molding and has easy component control and also provide a method for industrially efficiently producing the sponge titanium.

Means for Solving Problem

The above problems are solved by the present invention described below.

Namely, the present invention (1) provides sponge titanium produced by the Kroll method, in which the total of a chlorine content and a magnesium content is 350 ppm by mass or lower and a filling density is 1.65 g/cm³ to 1.95 g/cm³.

The present invention (2) provides the sponge titanium according to (1), in which an average particle diameter is 1.7 mm to 19.1 mm.

The present invention (3) provides the sponge titanium according to (1) or (2), in which a ratio of sponge titanium fine particles having a particle diameter of 0.84 mm or smaller is 0.8% by mass or lower.

The present invention (4) provides a method for producing sponge titanium by the Kroll method, the method comprising:

a reduction-separation step of determining an area of a reaction bath surface to be 2.5 m² or larger,

(i) determining an average feed rate A of titanium tetrachloride per unit area of the reaction bath surface calculated in accordance with Formula (1):

A=Average feed rate of titanium tetrachloride per minute (kg/minute)/Area of reaction bath surface (m²)  (1)

to be 2.8 kg/(minute·m²) to 4.0 kg/(minute·m²) for a period from start of feed of titanium tetrachloride to metal magnesium to generation of sponge titanium to a position corresponding to an upper limit position of a collection target in a crushing step, and

(ii) determining a total feed amount of titanium tetrachloride to be an amount in which a bottom part load index B of a sponge titanium block calculated in accordance with Formula (2):

B=Mass of sponge titanium block(t)/Area of disc or seat contacting lower side of sponge titanium block (m²)  (2)

is 3.5 t/m² to 5.5 t/m²; and

the crushing step of determining the upper limit position of the collection target to be a position in a range of 40% to 50% on a mass basis from the bottom of the sponge titanium block and cutting, grinding, and sieving the sponge titanium block lower than the upper limit position of the collection target serving as a collection target to obtain sponge titanium.

The present invention (5) provides a method for producing a titanium ingot or a titanium alloy ingot, the method comprising:

using the sponge titanium as described in any one of (1) to (3) as a melting raw material.

The (6) present invention provides a method for producing a titanium ingot or a titanium alloy ingot, the method comprising:

using the sponge titanium as described in (4) as a melting raw material.

Effect of the Invention

The present invention can provide sponge titanium for large ingot production that is difficult to cause the problems due to chloride inclusion at the time of melting production of the large ingot by the melting method not associated with the compression molding and has easy component control and also provide a method for industrially efficiently producing the sponge titanium.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view illustrating the existing state of chlorine in sponge titanium (a porosity of 40%).

FIG. 2 is a schematic view illustrating the existing state of chlorine in sponge titanium (a porosity of 20%).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The sponge titanium according to the present invention is sponge titanium produced by the Kroll method and the total of the chlorine content and the magnesium content is 350 ppm by mass or lower and the filling density is 1.65 g/cm³ to 1.95 g/cm³.

In the industrial production of the sponge titanium by the Kroll method, the steps mainly include the chlorination-distillation step, the reduction-separation step, the crushing step, and the electrolysis step. The sponge titanium according to the present invention is sponge titanium obtained by carrying out the reduction-separation step of generating sponge titanium produced by the Kroll method, that is, generating a sponge titanium block by adding titanium tetrachloride onto magnesium in a melted state to reduce titanium tetrachloride and subsequently eliminating magnesium chloride serving as a by-product and residual magnesium from the sponge titanium block by vacuum separation and the crushing step of cutting, crushing, and sieving the sponge titanium block obtained by carrying out the reduction-separation step to give sponge titanium having a desired particle diameter. Hereinafter, the sponge titanium block obtained by reducing titanium tetrachloride is also merely described as a sponge titanium block.

The total of the chlorine content and the magnesium content in the in sponge titanium according to the present invention in terms of atoms is 350 ppm by mass or lower and preferably 300 ppm by mass or lower. The sponge titanium having the total of the chlorine content and the magnesium content in terms of atoms in the sponge titanium in the above range can efficiently produce an ingot that is difficult to cause the problems due to chloride and has no poor casting surface at the time of melting production of a large titanium ingot or a large titanium alloy ingot by the melting method not associated with the compression molding. Hereinafter, the large titanium ingot and the large titanium alloy ingot are also collectively described as the large titanium ingot. The chlorine content and the magnesium content in the sponge titanium can be determined as follows. First, the measurement-target sponge titanium is ground to form a sponge titanium lot. Subsequently, in accordance with JIS H 1610-2008 “Titanium and titanium alloys Sampling methods”, a large sample is sampled from each of the sponge titanium lots and thereafter the large sample is reduced and separated to give four test samples having a mass of 250 g. Subsequently, the chlorine contents of the four test samples are measured in accordance with the silver nitrate titration method described in JIS H 1615-1997 and the average value of the four measured values is determined to be the chlorine content in the sponge titanium lot. Subsequently, the magnesium contents in the four test samples are measured in accordance with the atomic absorption method described in JIS H 1616-1995 and the average value of the four measured values is determined to be the magnesium content in the sponge titanium lot.

The filling density of the sponge titanium according to the present invention is 1.65 g/cm³ to 1.95 g/cm³ and preferably 1.70 g/cm³ to 1.95 g/cm³. The sponge titanium having the filling density in the above range allows the large titanium ingot to be difficult to cause deviation of the component specification due to local component concentration in the large titanium ingot at the time of melting production by the melting method not associated with the compression molding, that is, the component is easily controlled. In the component specification of the titanium ingot and the titanium alloy ingot, the content of iron and oxygen is required to be 250 ppm by mass or lower. For the large titanium ingot having a size of several tons or larger, the specification is required to be satisfied across the whole length. Specifically, when three positions or five positions located from the bottom end to the top end are analyzed, the predetermined component specification is required to be satisfied at all positions. However, the difficulty level of the requirement is high and deviation of the component specification due to the local component concentration in the large titanium ingot such as excess of the upper limit value only at one value that is the measurement value at the bottom or excess of the upper limit value only at one value that is the measurement value at the top frequently occurs. As an example, existence of sponge titanium fine particles having a particle diameter of 0.84 mm or smaller causes the local component concentration. The sponge titanium fine particles existing in the sponge titanium have an iron content of as high as more than 10 times and an oxygen content as high as more than 2 times as compared to the sponge titanium. Such sponge titanium fine particles do not cause a problem in the melting method in which a titanium raw material is compression-molded such as the consumption electrode type arc melting method. However, in the melting method not associated with compression molding of the raw material such as the electron beam melting method, phenomena of disproportionately supplying the sponge titanium fine particles and locally depositing the sponge titanium fine particles in a water-cooled copper hearth occur when the raw material is supplied from inside of a material feeding facility. Consequently, at the time of disproportionately supplying the sponge titanium fine particles or at the time of melting the locally deposited substance of the sponge titanium fine particles, the iron content and the oxygen content in the large titanium ingot becomes locally high. This leads to the deviation of the component specification of the titanium ingot.

Conventional sponge titanium allows the sponge titanium fine particles in an amount of 1.5% by mass to 2.5% by mass to be associated with the sponge titanium as a result of, for example, sponge titanium crush in the mixing step after sieving even if the sponge titanium is sieved using a sieve having a mesh size of 0.84 mm or larger. Similar to the above results, these sponge titanium fine particles generated by crushing have high iron content and oxygen content compared to the sponge titanium. In the sponge titanium, a part where a high iron or oxygen content locally exists and the part where the iron or oxygen content is high is more easily crushed than the part where the iron or oxygen content is low. Consequently, the conventional sponge titanium has easily generated the sponge titanium fine particles having high iron content and oxygen content. Therefore, even if the sponge titanium is sieved to eliminate the sponge titanium fine particles, the sponge titanium fine particles having high iron content and oxygen content are generated in the subsequent step. Therefore, the conventional sponge titanium has had a problem of local component concentration in the large titanium ingot due to the sponge titanium fine particles having high iron content and oxygen content. On the contrary, the sponge titanium according to the present invention has a high filling density of 1.65 g/cm³ to 1.95 g/cm³ and preferably 1.70 g/cm³ to 1.95 g/cm³ and thus the bulk density is high. Therefore, even if a part having high iron content and oxygen content locally exists, the part is difficult to crush. Therefore, the sponge titanium according to the present invention is difficult to generate the sponge titanium fine particles having high iron content and oxygen content and thus the problem of local component concentration in the large titanium ingot due to the sponge titanium fine particles having high iron content and oxygen content is difficult to arise. Consequently, the problem of local deviation of the component in the large titanium ingot is difficult to occur.

The filling density of the sponge titanium is an index representing the bulk density of the sponge titanium. In the present invention, the filling density refers to a density when the sponge titanium is filled into a 200 L drum. Specifically, the filling density of the sponge titanium is determined as described below. A steel open drum of Type D and Class M described in JIS Z1600-2006 is used as a measurement container. First, the inner diameter D (cm) and the inner height H (cm) (a distance from the upper surface of the drum bottom plate to the upper end of the drum) of the drum are measured. Subsequently, 100 kg or larger of the measurement-target sponge titanium is charged into the drum and the upper surface of the sponge titanium is smoothed so as to be flat. Subsequently, the distance from the upper end of the drum to the upper surface of the sponge titanium is measured with a ruler. This measurement is carried out at four different points in a radial direction in the drum (0°, 90°, 180°, and 270°) and the average value of the four measurement values is determined to be the distance between the upper end of the drum and the sponge C (cm). Subsequently, the filling density ρ (g/cm³) is calculated in accordance with Formula (3):

1/ρ=V/M=(π/M)·(D/2)²·(H−C)  (3)

using the inner diameter D (cm), the inner height H (cm), the distance between the upper end of the drum to the sponge C (cm), and the mass of the sponge titanium M (g). Here, V (cm³) is the filling volume of the sponge titanium. As the sponge titanium, sponge titanium having an average particle diameter in the range of 1.7 mm to 19.1 mm is used as the target.

In the sponge titanium according to the present invention, the ratio of the sponge titanium fine particles having a particle diameter of 0.84 mm or smaller is 0.8% by mass or lower, preferably 0.7% by mass or lower, and more preferably 0.4% by mass or lower. The sponge titanium having the ratio of the sponge titanium fine particles having a particle diameter of 0.84 mm or smaller within the above range allows the problem of local component concentration in the large titanium ingot due to the sponge titanium fine particles having high iron content and oxygen content to be difficult to occur and the problem of local deviation of the component in the large titanium ingot to be difficult to occur. In the present invention, as a method for determining the ratio of the sponge titanium fine particles having a particle diameter of 0.84 mm or smaller, the ratio of the sponge titanium fine particles having a particle diameter of 0.84 mm or smaller is determined by preparing the sponge titanium serving as the measurement target sampled in accordance with JIS H 1610:2008, and measuring particle size distribution of the sampled sponge titanium in accordance with JIS H 2151:2015. In the present invention, the sponge titanium having a particle diameter of 0.84 mm or smaller is defined as the sponge titanium fine particles.

The average particle diameter of the sponge titanium according to the present invention is 1.7 mm to 19.1 mm. In the present invention, as the method for measuring the average particle diameter of the sponge titanium, it is clear that the sponge titanium passing a sieve having a sieve opening of 19.1 mm or smaller and not passing a sieve having a sieve opening of 1.7 mm or smaller has the average particle diameter within a range of 1.7 mm to 19.1 mm and thus the measurement can be skipped. In the case where more accurate measurement is required, the measurement is carried out as follows. The measurement is carried out by preparing the sponge titanium serving as the measurement target sampled in accordance with JIS H 1610:2008, and passing a plurality of sieves having different sieve openings in accordance with JIS H 2151:2015. Specifically, the sieves having sieve openings of 1.7 mm, 4.75 mm, 12.7 mm, 19.1 mm, and 25.4 mm are used and mass percent of the sponge titanium group passing through each of the sieves is measured. Thereafter, the particle diameter of the group having a particle diameter of 1.7 mm or smaller is determined to be 0.85 mm; the particle diameter of the group having a particle diameter of 1.7 mm to 4.75 mm is determined to be 3.25 mm; the particle diameter of the group having a particle diameter of 4.75 mm to 12.7 mm is determined to be 8.73 mm; the particle diameter of the group having a particle diameter of 12.7 mm to 19.1 mm is determined to be 15.9 mm; and the particle diameter of the group having a particle diameter of 19.1 mm to 25.4 mm is determined to be 22.3 mm and the weighted average value of the measurement values is determined to be the average particle diameter.

A porosity ε of the sponge titanium according to the present invention before grinding is 20% to 50% and preferably 20% to 40%. The sponge titanium having the porosity within the above range allows the problem of local component concentration in the large titanium ingot due to the sponge titanium fine particles having high iron content and oxygen content to be difficult to occur and the problem of local deviation of the component in the large titanium ingot to be difficult to occur. In the present invention, that the porosity E before grinding is within the above range is determined as follows. First, each one sponge titanium sample having a mass of 100 g to 300 g (the diameter is about 50 mm to about 150 mm) is collected from three places of the proximity of the center of an axis A1, the proximity of circumference B1, and the proximity of circumference C1 that is located at a position shifted at 180 degrees from the proximity of the circumference B1 using the center of axis as the center. These collection points are in the vicinity of the upper limit position of the collection target of the sponge titanium block. The porosity of each of the samples is measured by the measurement method described below to determine the average value of the measured values to be the pore ratio in the vicinity of the upper limit position. Subsequently, each one sponge titanium sample having a mass of 100 g to 300 g is collected from three places of the proximity of the center of an axis A2, the proximity of circumference B2, and the proximity of circumference C2 that is located at a position shifted at 180 degrees from the proximity of the circumference B2 using the center of axis as the center. These collection points are in the vicinity of the upper limit position of the collection target of the sponge titanium block. The porosity of each of the samples is measured by the measurement method described below to determine the average value of the measured values to be the porosity in the vicinity of the lower limit position. In the case where both of the porosity E of the upper limit position of the collection target and the porosity E of the lower limit position of the collection target are within a range of 20% to 50%, the porosity E before grinding the collection target is determined to be within the range of 20% to 50%.

The range of the porosity ε′ of the sponge titanium according to the present invention after grinding is 5% to 50%, which is broader than the range of the porosity E before grinding. This is because particles that have smaller porosity due to extreme compression and particles of which the porosity is not changed due to no compression in the process of grinding are mixed in the same lot. When the porosity ε′ after grinding is determined, five or more sponge titanium particles larger than the average particle diameter are collected and the porosity of each of the samples is measured by the measurement method described below. The average value of the measured values is determined to be the porosity ε′ after grinding. The reason why the sponge titanium particles having a particle diameter larger than the average particle diameter are determined to be the target is because the accurate measurement of the porosity of the sample having extremely small particle diameter is difficult.

Here, the measurement method of the porosity will be described. First, mass W (g) of a sponge titanium sample is measured. Subsequently, paraffin (manufactured by Tissue Tech, Inc., paraffin wax) is melted by heating at about 80° C. to about 120° C. in a heat-resistant container. Subsequently, the sponge titanium sample is hung with a thread-like product and immersed into the paraffin solution. After checking no bubble appearance, the sample is gently pulled up and cooled with the sample hung in the air. Subsequently, a container containing water is prepared and placed on a weighing device. The cooled sponge titanium sample was gently immersed into water with the sample hung using the thread so as not to be contacted to the container. The amount of mass change (g) before and after the immersion is recorded using the weighing device. The amount of mass change (g) is divided by the density of water (g/cm³) to determine the apparent volume (cm³) of the sponge titanium. The bulk density ρ (g/cm³) is determined by dividing the mass W of the sponge titanium sample by the volume V (ρ=W/V). The porosity (%) is determined using the bulk density ρ in accordance with Formula (4):

Porosity(%)=(1−(ρ/4.51))×100  (4)

Chlorine existing as an impurity in the sponge titanium exists as the following three Types. In Type 1, chorine exists as magnesium chlorine in the fine pores in the primary particles of titanium. In Type 2, chlorine exists as magnesium chloride remaining in the gaps between the primary particles of titanium. In Type 3, chlorine exists as titanium dichloride attached onto the surfaces of the primary particles of titanium. Of these Types, Type 2 is the main existing state. In the cross section observation photograph of the sponge titanium sample illustrated in FIG. 1, a substance existing at spots in a part surrounded by the dotted line located in the upper side is magnesium chloride 1 of Type 1. In a part surrounded by the solid line located in the middle-lower side in FIG. 1, magnesium chloride 2 of Type 2 exists and void exists around this magnesium chloride. In a part surrounded by the solid line located in the middle-lower side in FIG. 2, magnesium chloride 2 of Type 2 exists. The magnesium chloride is enclosed with densely sintered primary particles of titanium. The cross section observation photographs of the sponge titanium sample illustrated in FIG. 1 and FIG. 2 are schematic photographs.

As a result of intensive studies of the relation between the amount of remaining these Types of chlorine and conditions and operations of the steps, the inventors of the present invention have found the followings. The existing state of chlorine in the sponge titanium is analyzed by embedding the sponge titanium sample serving as the measurement target into a resin, polishing the embedded samples with an emery paper of #1000, and observing the cross section with an electron beam microanalyzer (SUPERPROBE JXA-8100, manufactured by JEOL Ltd.). At this analysis, in order to prevent elution into water and moisture absorption of the chlorides, the sample is not in contact with the water until cutting-polishing-observation and operation is quickly carried out.

As a result of carrying out a plurality of tests, the inventors have confirmed that the existing state of chlorine observed in the sponge titanium at the upper part of the sponge titanium block obtained by carrying out the reduction-separation step is mainly Type 1 and the chloride is included in the titanium particles to disperse broadly and finely. Consequently, the chloride cannot be eliminated by volatilization in the vacuum separation. Therefore, it has been found that the chlorine content at the upper part of the sponge titanium block is as high as 1,000 ppm by mass to 1,500 ppm by mass.

On the contrary, the existing state observed in the sponge titanium at the lower part of the sponge titanium block is mainly Type 2. Magnesium chloride remains in the gaps between the primary particles of titanium. A part of Type 2 is completely enclosed in the sponge titanium due to dense sintering of the primary particles of titanium. In addition, a small amount of Type 1 is also observed. It has been found that the chlorine content at the lower part of the sponge titanium block is about 400 mm by mass to about 600 ppm by mass in total of Type 2 and Type 1.

From these results, it has been found that reduction in chlorine of Type 1 and Type 2 is important in order to reduce the chlorine content in the sponge titanium.

As can be seen from the observation result of the upper part of the sponge titanium block, it is assumed that the amount of generated Type 1 is increased when the feed rate of metal magnesium becomes insufficient relative to the feed rate of titanium tetrachloride at the reaction site. The feed rate of metal magnesium described herein refers to a rate of submerging magnesium chloride generated as the by-product at the reaction bath surface into the bath, floating magnesium in the bath, and feeding the floated magnesium to the reaction bath surface instead of magnesium chloride due to specific gravity difference. Therefore, it has been assumed that appropriate control of the ratio of the feed rate of titanium tetrachloride and the feed rate of metal magnesium to the reaction site is important in order to reduce Type 1 chlorine.

With respect to Type 2, attention is paid to Type 2 magnesium chloride completely enclosed between the primary particles of titanium (hereinafter called enclosed type Type 2) due to dense sintering of the primary particles of titanium. As a result of further continuous study, it has been found that “generation frequency of the enclosed type Type 2 becomes higher when the porosity of the sponge titanium becomes smaller to some extent” and “a high chlorine-containing site having a chlorine content of higher than 1,000 ppm by mass may be generated by the enclosed type Type 2 even at the lower part of the sponge titanium block”. In addition, it has been found that the porosity ε is mainly dominated by sintering the primary particles of titanium with each other at the time of vacuum separation in the reduction-separation step. Therefore, in order to reduce the chlorine content, it has been assumed that reduction in the generation frequency of enclosed type Type 2 is effective by preventing excessively small porosity formation of the sponge titanium. On the other hand, it has been assumed that a small porosity is desirable in order to be difficult to cause the problem of local component concentration in the large titanium ingot due to the sponge titanium fine particles having high iron content and oxygen content by increasing the filling density and making the part where the iron content and the oxygen content are locally high be difficult to crush.

From these studies, for reduction in Type 2, it has been found that control of the porosity of the sponge titanium in an appropriate range by appropriately controlling the sintering conditions of the sponge titanium is important. The inventors of the present invention have found that excessively large compression load results in excessively small porosity of the sponge titanium to easily generate enclosed type Type 2 and thus the compression load is required to be adjusted in an apocopate range.

Based on these findings, the inventors of the present invention have conceived a method for producing the sponge titanium according the present invention described below.

The method for producing the sponge titanium according to the present invention is a method for producing the sponge titanium by the Kroll method, the method comprising:

a reduction-separation step of determining an area of a reaction bath surface to be 2.5 m² or larger,

(i) determining an average feed rate A of titanium tetrachloride per unit area of the reaction bath surface calculated in accordance with Formula (1):

A=Average feed rate of titanium tetrachloride per minute (kg/minute)/Area of reaction bath surface (m²)  (1)

to be 2.8 kg/(minute·m²) to 4.0 kg/(minute·m²) for a period from start of feed of titanium tetrachloride to metal magnesium to generation of sponge titanium to a position corresponding to an upper limit position of a collection target in a crushing step, and

(ii) determining a total feed amount of titanium tetrachloride to be an amount in which a bottom part load index B of a sponge titanium block calculated in accordance with Formula (2):

B=Mass of sponge titanium block(t)/Area of disc or seat contacting lower side of sponge titanium block (m²)  (2)

is 3.5 t/m² to 5.5 t/m²; and

the crushing step of determining the upper limit position of the collection target to be a position in a range of 40% to 50% on a mass basis from the bottom of the sponge titanium block and cutting, grinding, and sieving the sponge titanium block lower than the upper limit position of the collection target serving as a collection target to obtain sponge titanium.

The method for producing the sponge titanium according to the present invention is a method for producing sponge titanium by the Kroll method and includes the reduction-separation step of generating a sponge titanium block by adding titanium tetrachloride onto metal magnesium in a melted state to reduce titanium tetrachloride and subsequently eliminating magnesium chloride serving as a by-product and residual magnesium from the sponge titanium block by vacuum separation and the crushing step of cutting, crushing, and sieving the sponge titanium block obtained by carrying out the reduction-separation step to give sponge titanium having a desired particle diameter.

In the reduction-separation step according to the method for producing the sponge titanium according to the present invention, first, titanium tetrachloride is fed onto a metal magnesium bath in the reaction container and the metal magnesium is reacted with the titanium tetrachloride to reduce titanium tetrachloride. During this step, main reaction occurs at the reaction bath surface in the reaction container and in the space above the reaction bath surface to generate primary particles of titanium. Metal magnesium in the vicinity of the reaction bath surface is consumed and magnesium chloride is generated as the by-product. The generated primary particles of titanium are submerged downward in the reaction container and accumulated onto a disc or a seat located at the bottom of the reaction container. The specific gravity of metal magnesium is smaller than that of magnesium chloride and thus magnesium chloride generated as the by-product is submerged downward in the reaction container and metal magnesium is floated instead of magnesium chloride. During carrying out the reduction reaction, the submerged magnesium chloride is appropriately taken out at the lower side of the reaction container. However, the submerged magnesium chloride is unable to be taken out completely. Therefore, after completion of the reduction reaction, both of the residual magnesium chloride and the unreacted metal magnesium remain in the sponge titanium block.

In the reduction-separation step, the residual magnesium chloride and the unreacted metal magnesium are eliminated from the sponge titanium block by vacuum separation. At the vacuum separation, the reaction container containing the generated sponge titanium block and a vacant reaction container are adjacently disposed and the upper parts of both of the reaction containers are connected to each other with a piping. Vacuuming the inside of the latter reaction container while the former reaction container is being heated from outside allows the metal magnesium and the magnesium chloride included in the sponge titanium block in the former reaction container to be transferred to the vacant reaction container in a gas state through the piping connecting the upper parts of the reaction containers to each other. The metal magnesium transferred to the vacant reaction container is used in the reduction step again.

In the method for producing the sponge titanium according to the present invention, in the reduction-separation step, the area of the reaction bath surface is determined to be 2.5 m² or larger and, when titanium tetrachloride is fed onto the metal magnesium bath, titanium tetrachloride is fed onto the metal magnesium bath under conditions of (i) determining an average feed rate A of titanium tetrachloride per unit area of the reaction bath surface calculated in accordance with Formula (1):

A=Average feed rate of titanium tetrachloride per minute (kg/minute)/Area of reaction bath surface (m²)  (1)

to be 2.8 kg/(minute·m²) to 4.0 kg/(minute·m²), and preferably 2.8 kg/(minute·m²) to 3.6 kg/(minute·m²) for the period from start of feed of titanium tetrachloride to metal magnesium to generation of sponge titanium up to the position corresponding to the upper limit position of the collection target in the crushing step. The total of the chlorine content and the magnesium content can be reduced to 350 ppm by mass or lower and preferably 300 ppm by mass or lower by determining the area of the reaction bath surface to be 2.5 m² or larger and determining the average feed rate A of titanium tetrachloride per unit area of the reaction bath surface to be 2.8 kg/(minute·m²) to 4.0 kg/(minute·m²), and preferably 2.8 kg/(minute·m²) to 3.6 kg/(minute·m²) for the period from start of feed of titanium tetrachloride to metal magnesium to generation of sponge titanium up to the position corresponding to the upper limit position of the collection target in the crushing step.

Here, in the method for producing the sponge titanium according to the present invention, first, before the feed of titanium tetrachloride into the reaction container, the upper limit position of the collection target in the crushing step is determined to be a position at what percent on a mass basis from the bottom of the sponge titanium block. Subsequently, the time point of sponge titanium generation up to the upper limit position of the collection target in the crushing step when titanium tetrachloride is fed into the reaction container in the theoretical amount of titanium tetrachloride required for generating the sponge titanium to the position is grasped. For example, in the case where the upper limit position of the collection target in the crushing step is determined to be a position at 50% on a mass basis from the bottom of the sponge titanium block, the period from the start of feed of titanium tetrachloride to metal magnesium to generation of sponge titanium up to the position corresponding to an upper limit position of a collection target in the crushing step is defined as a period from the start of feed of titanium tetrachloride to metal magnesium in a melted state to the termination of the feed of titanium tetrachloride in 50% of total titanium tetrachloride to be fed to the reaction container. In addition, for example, in the case where the upper limit position of the collection target in the crushing step is determined to be a position at 40% on a mass basis from the bottom of the sponge titanium block, the period from the start of feed of titanium tetrachloride to metal magnesium to generation of sponge titanium up to the position corresponding to an upper limit position of a collection target in the crushing step is defined as a period from the start of feed of titanium tetrachloride to metal magnesium in a melted state to the termination of the feed of titanium tetrachloride in 40% of total titanium tetrachloride to be fed to the reaction container.

The feed rate of titanium tetrachloride per unit area of the reaction bath surface during generation of the sponge titanium at the position corresponding to the position upper than the upper limit position of the collection target in the crushing step is appropriately selected depending on production efficiency of the sponge titanium. In general, the feed rate is selected from a range of 1.0 kg/(minute·m²) to 5.5 kg/(minute·m²). As the reaction becomes closer to its end, the metal magnesium in the reaction container becomes less. Therefore, the feed rate of titanium tetrachloride is preferably set to a lower value close to the end of the reaction.

The area of the reaction bath surface refers to the area of the upper surface of metal magnesium in a melted state in the reaction container and corresponds to the area of the horizontal cross section of the reaction container at the position of the upper surface of the metal magnesium in the container.

In the method for producing the sponge titanium according to the present invention, in the reduction-separation step, when titanium tetrachloride is fed onto the metal magnesium bath, (ii) a total feed amount of titanium tetrachloride is determined to be an amount in which a bottom part load index B of a sponge titanium block calculated in accordance with Formula (2):

B=Mass of sponge titanium block(t)/Area of disc or seat contacting lower side of sponge titanium block (m²)  (2)

is 3.5 t/m² to 5.5 t/m² and preferably 4.0 t/m² to 5.5 t/m². The total feed amount of titanium tetrachloride is determined to be an amount of the bottom part load index B of the sponge titanium block of 3.5 t/m² to 5.5 t/m² and preferably 4.0 t/m² to 5.5 t/m², whereby the porosity ε can be 20% to 50% and preferably 20% to 40%. Consequently, the filling density of the sponge titanium can be led to 1.65 g/cm³ to 1.95 g/cm³ and preferably 1.70 g/cm³ to 1.95 g/cm³. In the case where the bottom part load index B of the sponge titanium block obtained by carrying out the reduction-separation step is larger than the above range, the porosity is small, the bulk density is high, and the filling density is high because the compression load applied to the sponge titanium becomes large during the vacuum separation. However, the porosity of the sponge titanium becomes excessively small and the amount of enclosed type Type 2 magnesium chloride is increased, resulting in a high chlorine content in the sponge titanium because the compression load applied to the sponge titanium becomes excessively large. On the other hand, in the case where the bottom part load index B of the sponge titanium block obtained by carrying out the reduction-separation step is smaller than the above range, the porosity is excessively high because the compression load applied to the sponge titanium becomes excessively small during the vacuum separation. Consequently, the bulk density is lower than the above range and thus the filling density is lower than the above range.

The area of the disc or seat contacting the lower side of the sponge titanium block refers to the area of the upper surface of the disc in the case where the seat and the disc placed on the seat are disposed under the sponge titanium block and refers to the area of the upper surface of the seat in the case where the disc is not placed and the seat alone is disposed under the sponge titanium block. The mass of the sponge titanium block at the time of calculating the B value refers to a theoretical value calculated by assuming that all titanium tetrachloride fed into the reaction container is converted into metal titanium and is a value calculated by multiplying the whole number of moles of titanium tetrachloride fed into the reaction container by the atomic weight of titanium.

Conditions in the reduction reaction in the reduction-separation step other than the above conditions such as a temperature outside of the reaction container in the reduction reaction may be usual conditions used in the production of the sponge titanium by the Kroll method and the temperature of the reduction reaction container is 700° C. to 950° C.

The vacuum heating temperature in the vacuum separation in the reduction-separation step is not particularly limited and is preferably 900° C. to 1,080° C. The vacuum heating time in the vacuum separation in the reduction-separation step is not particularly limited and time when magnesium chloride and metal magnesium that can be separated by volatilization are eliminated is appropriately selected. As described above, Type 1 and enclosed type Type 2 chlorine exist so as to be enclosed in the sponge titanium and thus the chlorine cannot be eliminated even when the vacuum heating time is elongated. Therefore, wastefully long vacuum heating time wastes time and energy and thus the vacuum heating time may be the time when magnesium chloride and metal magnesium that can be separated by volatilization is eliminated. From the viewpoint of carrying out the vacuum separation without wasting time and electric power, the vacuum heating time during the vacuum separation in the reduction-separation step is preferably in the range satisfying “112≤0.26×r−p≤125” where the vacuum heating time is p (hour) and the radius of the disc or seat contacting the lower part of the sponge titanium block is r (mm) (with the proviso that r is 600 mm or longer). In the case where the upper surface of the disc or the seat is a perfect circle, the radius of the perfect circle is determined to be r and in the case where the upper surface of the disc or the seat is an oval, the average value of the maximum radius and the minimum radius is determined to be r.

In the crushing step according to the method for producing the sponge titanium according to the present invention, the upper limit position of the collection target is determined to be the position in the range of 40% to 50% on a mass basis from the bottom of the sponge titanium block. The sponge titanium block below the upper limit position of the collection target is cut, ground, and sieved as the collection target to give the sponge titanium. The upper limit position of the collection target in the crushing step is a position that is determined to be the upper limit position of the collection target in the crushing step before feeding the titanium tetrachloride into the reaction container. The position of X % on a mass basis from the bottom of the sponge titanium block refers to a position where the accumulated mass reaches X % on a mass basis relative to the whole mass of the sponge titanium block when titanium located in the same height is accumulated in the vertical direction from the bottom of the sponge titanium block. The collection target site in the present invention may be acceptable even when the site is a part of a range such as 5% to 30% or 20% to 35% as long as the site is located in the lower range from the upper limit position.

In the crushing step, the sponge titanium block obtained by carrying out the reduction-separation step is taken out from the reaction container using a known push-out apparatus, cut in a round slice shape and roughly crushed by a known large press machine, and separated for each part. The roughly crushed small mass sponge titanium is ground in a size of 100 mm or smaller by a known shear or the like to give the sponge titanium. At this time, the sponge titanium block existing below the position that is determined to the upper limit position of the collection target is cut, ground, and sieved as the collection target. The upper limit position of the collection target is determined to be within the range of 40% to 50% on a mass basis from the bottom of the sponge titanium block and the sponge titanium block existing blow the upper limit position of the collection target is cut, ground, and sieved to give the sponge titanium having a filling density of 1.65 g/cm³ to 1.95 g/cm³, preferably 1.70 g/cm³ to 1.95 g/cm³. In addition, the upper side of the sponge titanium block, in which the chlorine content tends to be high due to the high content of Type 1 chlorine, is avoided and the lower side of the sponge titanium block having low chlorine content due to the low content of Type 1 chlorine is used as the collection target. Consequently, the chlorine content in the obtained sponge titanium can be reduced.

The proximity of the low end of the sponge titanium block obtained by carrying out the reduction-separation step is sponge titanium generated during the initial stage of the reaction and does not satisfy the common component specification due to concentration of iron, nitrogen, aluminum, and nickel in metal magnesium serving as the raw material. Therefore, the sponge titanium in the proximity of the low end of the sponge titanium block obtained by carrying out the reduction-separation step is not used in the crushing step as the collection target. In the crushing step, the position of what percent from the bottom of the sponge titanium block is determined to be the lower limit of the collection target is appropriately selected depending on the chlorine content and the magnesium content in the sponge titanium. The position of 2% to 8% on a mass basis from the bottom of the sponge titanium block is preferably determined to be the lower limit of the collection target of the sponge titanium and the position of 2% to 5% on a mass basis from the bottom of the sponge titanium block is particularly preferably determined to be the lower limit of the collection target of the sponge titanium.

The sponge titanium according to the present invention and the sponge titanium obtained by carrying out the method for producing sponge titanium according to the present invention have remarkably low chloride content and thus occurrence of problems caused by including chlorides such as generation of large amount of splash associated with volatilization of the chlorides and worsening of yield in melting in the melting method not associated with compression molding, impossibility of raw material feeding due to attaching or depositing the splash to the raw material feed opening, inhibition of generation of electron beam due to the volatized chloride, and corrosion of melting facilities due to generated preclude gas can be prevented. The sponge titanium according to the present invention and the sponge titanium obtained by carrying out the method for producing sponge titanium according to the present invention have remarkably low chloride content and thus occurrence of the problem of poor casting surface in the melting method not associated with compression molding can be prevented. The sponge titanium according to the present invention and the sponge titanium obtained by carrying out the method for producing sponge titanium according to the present invention have a filling density of 1.65 g/cm³ to 1.95 g/cm³ and preferably 1.70 g/cm³ to 1.95 g/cm³ and thus the problem of local component concentration in the large titanium ingot due to the sponge titanium fine particles having high iron content and oxygen content and generated by crushing the part having high iron content and oxygen content can be prevented and thus the problem of the local deviation of the component in the large titanium ingot can be prevented.

The method for producing the sponge titanium according to the present invention can provide sponge titanium having a total of the chlorine content and the magnesium content of 350 ppm by mass or lower and preferably 300 ppm by mass or lower, and a filling density of 1.65 g/cm³ to 1.95 g/cm³ and preferably 1.70 g/cm³ to 1.95 g/cm³ by determining the range of the collection target in the crushing step to be the lower side from the position of 40% to 50% on a mass basis from the bottom of the sponge titanium block, determining the feed rate of the titanium tetrachloride during generation of the sponge titanium in the range of the collection target to be 2.8 kg/(minute·m²) to 4.0 kg/(minute·m²) and preferably 2.8 kg/(minute·m²) to 3.6 kg/(minute·m²) and determining the area of the reaction bath surface to be 2.5 m² or larger, determining the bottom part load index of the sponge titanium block to be 3.5 t/m² to 5.5 t/m² and preferably 4.0 t/m² to 5.5 t/m² by adjusting the total feed amount of titanium tetrachloride during the reduction reaction, and thus achieving the porosity of the collection target part of the titanium tetrachloride to be 20% to 50% and preferably 20% to 40%.

The method for producing the titanium ingot or the titanium alloy ingot according to the first aspect of the present invention is a method for producing a titanium ingot or a titanium alloy ingot including using the sponge titanium according to the present invention as a melted raw material. The method for producing the titanium ingot or the titanium alloy ingot according to the second aspect of the present invention is a method for producing a titanium ingot or a titanium alloy ingot including using the sponge titanium obtained by carrying out the method for producing the sponge titanium according to the present invention as a melted raw material.

Hereinafter, the present invention will be further specifically described with reference to Examples. However, Examples are merely exemplification and the present invention is not limited thereto.

EXAMPLE Example 1

Sponge titanium was produced using an 8-ton batch production apparatus having a sponge titanium standard generation amount per batch of 8.0 tons, a container cross section area of a reaction bath surface part of 2.5 (m²), and a radius r of a disc of 750 (mm).

First, 10 t of melted magnesium was charged in the production apparatus and titanium tetrachloride was fed into the production apparatus to carrying out reduction reaction of titanium tetrachloride. During the reduction reaction, for the feed of 0 (t) to 16 (t) of titanium tetrachloride, which corresponded to the zone of the former half of 50% of 32 (t) of the total feed amount of titanium tetrachloride, titanium tetrachloride was fed by controlling the feed rate so that the average feed rate A of titanium tetrachloride per unit area of the reaction bath surface was 3.3 (kg/(minute·m²) and subsequently 16 (t) to 32 (t) of titanium tetrachloride, which corresponded to the latter half of 50% was fed. The bottom part load index B is 4.5 (t/m²).

Subsequently, vacuum separation was carried out to give a sponge titanium block. At this time, vacuum heating was carried out under conditions in which the vacuum heating temperature was 1050° C. and the vacuum heating time p (hour) satisfied 0.26×r−p=122.

Subsequently, the obtained sponge titanium block was sequentially cut from the lower side. First, 400 kg of the low quality sponge titanium block, which corresponded to 5% in 8.0 tons on a mass basis, was cut and eliminated. Subsequently, 3.6 tons of the sponge titanium block, which corresponded to 5% to 50% in 8.0 tons on a mass basis, was collected from the lower side of the sponge titanium block separating from the other site. At this time, three sponge titanium samples having a mass of 100 g to 300 g were collected from the proximity of the center of the axis A1, the proximity of circumference B1, and the proximity of circumference C1 that was located at a position shifted at 180 degrees from the proximity of the circumference B1 using the center of axis as the center. These collection points were in the vicinity of the upper limit position of the collection target of the sponge titanium block. The porosities of these samples were determined and these porosities were averaged. The porosity ε at the upper limit position of the collection target before grinding was 47%. In addition, three sponge titanium samples were collected from the proximity of the center of the axis A2, the proximity of circumference B2, and the proximity of circumference C2 that was located at a position shifted at 180 degrees from the proximity of the circumference B2 using the center of axis as the center. These collection points were in the vicinity of the lower limit position of the collection target of the sponge titanium block. The porosities of these samples were determined and these porosities were averaged. The porosity ε at the lower limit position of the collection target before grinding was 23%.

Subsequently, the sponge titanium other than the sample of which the porosity was measured was ground by a shear or the like and sieved so that the size of the crushed sponge titanium was smaller than a sieve opening of a sieve of 19.1 mm and larger than a sieve opening of a sieve of 0.84 mm. Thereafter the sieved sponge titanium was made to be uniform by a mixer to give Sponge titanium A.

Subsequently, the chlorine content, the magnesium content, and the filling density of Sponge titanium A were measured. The chlorine content was 200 ppm by mass, the magnesium content was 90 ppm by mass, and the filling density was 1.70 g/cm³. A fine particle product having a size of 0.84 mm or smaller was contained in 0.2% by mass. In addition, the 5 particles having a size of larger than 1.27 mm were randomly selected from Sponge titanium A and the porosities were determined and averaged. The porosity ε′ after grinding was 25%. The results are listed in Table 1.

Example 2 and Comparative Examples 1 and 2

Sponge titanium was prepared by the same method as the method in Example 1 except that the zone where the average feed rate of titanium tetrachloride per unit area was controlled at 3.3 (kg/(minute·m²)) and the collection target site of the sponge titanium were determined to be the range listed in Table 1. The results are listed in Table 1.

TABLE 1 Comparative Comparative Example 1 Example 2 Example 1 Example 2 Collection target site of sponge titanium 5 to 50 5 to 40 5 to 60 5 to 70 (position on mass basis from bottom of sponge titanium block (%)) Feed rate control zone of titanium 0 to 50 0 to 40 0 to 60 0 to 70 tetrachloride (ratio to total feed amount of titanium tetrachloride (%)) Average feed rate of titanium tetrachloride 3.3 3.3 3.3 3.3 per unit area of reaction bath surface in feed rate control zone of titanium tetrachloride (kg/(minute · m²)) Porosity of collection target site before grinding Upper limit position 47% 36% 58% 59% Lower limit position 23% 21% 22% 21% Bottom part load index B (t/m²) 4.5 4.5 4.5 4.5 Sponge titanium (ppm by mass) Chlorine content 200 150 510 640 Magnesium content 90 60 150 120 Cl content + Mg content 290 210 660 760 Filling density of sponge titanium (g/cm³) 1.70 1.75 1.55 1.50 Fine particle amount in sponge titanium 0.2 0.2 1.4 1.5 (% by mass) Porosity of sponge titanium after grinding 25% 19% 32% 36%

As can be seen from Table 1, the chlorine content can be 290 ppm by mass or lower and the filling density can be 1.70 g/cm³ or higher by determining the collection target site to be lower from the position of 50% on a mass basis from the bottom of the sponge titanium block.

Examples 3 and 4 and Comparative Example 3

Sponge titanium was prepared by the same method as the method in Example 1 except that the average feed rate of 0 (t) to 16 (t) of titanium tetrachloride, which corresponded to the zone of the former half of 50% of 32 (t) of the total feed amount of titanium tetrachloride, per unit area of the reaction bath surface was determined to be the value listed in Table 2. The results are listed in Table 2.

Comparative Example 4

Sponge titanium was produced using a 6-ton batch production apparatus having a sponge titanium standard generation amount per batch of 6.0 tons, a container cross section area of a reaction bath surface part of 2.2 (m²), and a radius r of a disc of 700 (mm).

First, 7.5 t of melted magnesium was charged in the production apparatus and titanium tetrachloride was fed into the production apparatus to carrying out reduction reaction of titanium tetrachloride. During the reduction reaction, for the feed of 0 (t) to 12 (t) of titanium tetrachloride, which corresponded to the zone of the former half of 50% of 24 (t) of the total feed amount of titanium tetrachloride, titanium tetrachloride was fed by controlling the feed rate so that the average feed rate A of titanium tetrachloride per unit area of the reaction bath surface was 2.8 (kg/(minute·m²)) and subsequently 12 (t) to 24 (t) of titanium tetrachloride, which corresponded to the latter half of 50% was fed. The bottom part load index B is 4.0 (t/m²).

Subsequently, vacuum separation was carried out to give a sponge titanium block. At this time, vacuum heating was carried out under conditions in which the vacuum heating temperature was 1050° C. and the vacuum heating time p (hour) satisfied 0.26×r−p=120.

The following operations were carried out by the same method as the method in Example 1. The results are listed in Table 1.

TABLE 2 Comparative Comparative Example 3 Example 4 Example 3 Example 4 Collection target site of sponge titanium 5 to 50 5 to 50 5 to 50 5 to 50 (position on mass basis from bottom of sponge titanium block (%)) Feed rate control zone of titanium 0 to 50 0 to 50 0 to 50 0 to 50 tetrachloride (ratio to total feed amount of titanium tetrachloride (%)) Average feed rate of titanium tetrachloride 2.8 4.0 4.4 2.8 per unit area of reaction bath surface in feed rate control zone of titanium tetrachloride (kg/(minute · m²)) Area of reaction bath surface (m²) 2.5 2.5 2.5 2.2 Bottom part load index B (t/m²) 4.5 4.5 4.5 4.0 Porosity of collection target site before grinding Upper limit position 44% 46% 43% 49% Lower limit position 23% 23% 21% 27% Sponge titanium (ppm by mass) Chlorine content 160 210 440 620 Magnesium content 50 120 140 220 Cl content + Mg content 210 330 580 840 Filling density of sponge titanium (g/cm³) 1.70 1.65 1.70 1.60 Fine particle amount in sponge titanium 0.2 0.7 2.0 1.1 (% by mass) Porosity of sponge titanium after grinding 24% 24% 22% 28%

As can be seen from Table 2, the chlorine content can be reduced to 330 ppm by mass by reducing the feed rate of titanium tetrachloride per unit area of the reaction bath surface to 4.0 (kg/(minute·m²)) or lower in the zone of the former half of 50% of the total feed amount of titanium tetrachloride.

As can be seen from comparison of Example 3 and Comparative Example 4 in Table 2, the chlorine content becomes higher when the area of the reaction bath surface is smaller than 2.5 (m²), even when the average feed rates of titanium tetrachloride per unit area of the reaction bath surface are the same.

Examples 6 and 7 and Comparative Examples 5 and 6

In Example 6 and Comparative Example 5, sponge titanium was prepared by the same method as the method in Example 1 except that the bottom part load index indicating the total feed amount of titanium tetrachloride was changed according to the value listed in Table 3.

In Example 7 and Comparative Example 6, sponge titanium was prepared by the same method as the method in Example 1 except that the sponge titanium was produced using an 8.0-ton batch production apparatus having a sponge titanium standard generation amount of 12.0 tons per batch, a container cross section area of the reaction bath surface part of 3.5 (m²), and a radius r of a disc of 800 (mm) and the bottom part load index indicating the total feed amount of titanium tetrachloride was changed according to the value listed in Table 3.

TABLE 3 Comparative Comparative Example 5 Example 6 Example 7 Example 6 Total feed amount of titanium 21 25 44 48 tetrachloride (t) Collection target site of sponge titanium 5 to 50 5 to 50 5 to 50 5 to 50 (position on mass basis from bottom of sponge titanium block (%)) Feed rate control zone of titanium 0 to 50 0 to 50 0 to 50 0 to 50 tetrachloride (ratio to total feed amount of titanium tetrachloride (%)) Average feed rate of titanium tetrachloride 3.3 3.3 3.3 3.3 per unit area of reaction bath surface in feed rate control zone of titanium tetrachloride (kg/(minute · m²)) Area of reaction bath surface (m²) 2.5 2.5 3.5 3.5 Bottom part load index B (t/m²) 3.0 3.5 5.5 6.0 Porosity of collection target site before grinding Upper limit position 59% 48% 43% 39% Lower limit position 30% 28% 21% 18% Sponge titanium (ppm by mass) Chlorine content 190 180 200 1140 Magnesium content 60 70 50 330 Cl content + Mg content 250 250 250 1470 Filling density of sponge titanium (g/cm³) 1.60 1.65 1.90 2.00 Fine particle amount in sponge titanium 1.1 0.4 0.1 0.1 (% by mass) Porosity of sponge titanium after grinding 31% 22% 18% 15%

As can be seen from Table 3, a chlorine content of 290 ppm by mass or lower and a filling density of 1.65 g/cm³ to 1.90 g/cm³ can be achieved by controlling the bottom part load index and controlling the porosity of the collection target site in the range of 20% to 50%. In contrast, in Comparative Example 6 in which the porosity is lower than 20%, although the filling density can be achieved in a remarkably high value of 2.00 g/cm³, the chlorine content is extremely high as 1,400 ppm by mass, which is noncompliant.

Examples 8 and 9 and Comparative Examples 7 and 8

The sponge titanium produced by the same method as the method in each of Example 2, Example 5, Comparative Example 1, and Comparative Example 2 was used as a raw material. A deposited amount due to splash, a number of temporary stop times of the electron gun, and a production rate of titanium ingots were examined when four JIS type 1 titanium ingots having a weight of 10 t were produced by the electron beam method. The deposited amount due to splash was relatively compared to the case of Example 2 determined as 1 by observing a thickness of the deposited splash at the edge part of a water-cooled copper hearth from the observation window of the melting furnace. The production rate of the titanium ingots was relatively compared to the case of Example 2 determined as 1 by calculation by dividing the mass of the titanium ingots by the time from the charge of the titanium raw material into the melting furnace to the termination of flow of melted titanium to a casting mold. In addition, the casting surface and the component of the ingots produced by melting were evaluated. As the casting surface, the number of poor casting surface having a depth of 5 mm or deeper at the ingot surface was determined. As the component, the ingot was equally cut along the whole length from the bottom end to the top end at five points to compare the process capability index Cpk of iron and oxygen concentration when the component was analyzed. Cpk was calculated in accordance with Mathematical Formula (1). Here, USL is the upper limit value of a specification, LSL is the lower limit value of a specification, and σ is a standard deviation.

$\begin{matrix} {{Cpk} = {\min \left\lbrack {\frac{{USL} - \mu}{3\sigma},\frac{\mu - {LSL}}{3\sigma}} \right\rbrack}} & \left\lbrack {{Mathematical}\mspace{14mu} {Formula}\mspace{14mu} (1)} \right\rbrack \end{matrix}$

TABLE 4 Comparative Comparative Example 8 Example 9 Example 7 Example 8 Type of sponge titanium Example 2 Example 4 Comparative Comparative Example 1 Example 2 Sponge titanium (ppm by mass) Chlorine content 150 210 510 640 Magnesium content 60 120 150 120 Cl content + Mg content 210 330 660 760 Filling density of sponge titanium (g/cm³) 1.75 1.65 1.55 1.50 Fine particle amount in sponge titanium 0.2 0.7 1.4 1.5 (% by mass) Deposited amount due to splash (—) 1.0 1.4 2.5 3.3 Number of temporary stop times of 0 3 31 45 electron gun (times) Ingot production rate (—) 1.0 0.97 0.91 0.88 Counted number of poor casting surface 6 10 32 29 having depth of 5 mm or deeper (Count) Cpk of iron (—) 1.3 1.1 0.8 0.8 Cpk of oxygen (—) 0.9 0.8 0.6 0.6

As can be seen from Table 4, not only the indices related with melting production cost such as the deposited amount due to splash, the number of temporary stop times of the electron gun, and the production rate of the ingot but also the indices related with quality of the ingot such as the casting surface and Cpk of iron and oxygen are improved by using the sponge titanium having a chlorine content of 330 ppm by mass or lower and a filling density of 1.65 g/cm³ or higher serving as the melting raw material. 

1-6. (canceled)
 7. A method for producing sponge titanium by Kroll method, the method comprising: a reduction-separation step of determining an area of a reaction bath surface to be 2.5 m² or larger, (i) determining an average feed rate A of titanium tetrachloride per unit area of the reaction bath surface calculated in accordance with Formula (1): A=Average feed rate of titanium tetrachloride per minute (kg/minute)/Area of reaction bath surface (m²)  (1) to be 2.8 kg/(minute·m²) to 4.0 kg/(minute·m²) for a period from start of feed of titanium tetrachloride to metal magnesium to generation of sponge titanium to a position corresponding to an upper limit position of a collection target in a crushing step, and (ii) determining a total feed amount of titanium tetrachloride to be an amount in which a bottom part load index B of a sponge titanium block calculated in accordance with Formula (2): B=Mass of sponge titanium block(t)/Area of disc or seat contacting lower side of sponge titanium block (m²)  (2) is 3.5 t/m² to 5.5 t/m²; and the crushing step of determining the upper limit position of the collection target to be a position in a range of 40% to 50% on a mass basis from the bottom of the sponge titanium block and cutting, grinding, and sieving the sponge titanium block lower than the upper limit position of the collection target serving as a collection target to obtain sponge titanium.
 8. The method for producing sponge titanium by the Kroll method according to claim 7, wherein the average feed rate A of titanium tetrachloride per unit area of the reaction bath surface is 2.8 kg/(minute·m²) to 3.6 kg/(minute·m²).
 9. The method for producing sponge titanium by the Kroll method according to claim 7, wherein the bottom part load index B of the sponge titanium block is 4.0 t/m² to 5.5 t/m².
 10. The method for producing sponge titanium by the Kroll method according to claim 7, wherein a lower limit position of the collection target is determined to be a position of 2% to 8% on a mass basis from the bottom of the sponge titanium block.
 11. The method for producing sponge titanium by the Kroll method according to claim 10, wherein a lower limit position of the collection target is determined to be a position of 2% to 5% on a mass basis from the bottom of the sponge titanium block.
 12. The method for producing sponge titanium by the Kroll method according to claim 7, wherein a temperature of a reduction reaction container in reduction reaction in the reduction-separation step is 700° C. to 950° C.
 13. The method for producing sponge titanium by the Kroll method according to claim 7, wherein a vacuum heating temperature in vacuum separation in the reduction-separation step is 900° C. to 1,080° C.
 14. The method for producing sponge titanium by the Kroll method according to claim 7, wherein a vacuum heating time in vacuum separation in the reduction-separation step satisfies Formula (3) 112≤0.26×r−p≤125  (3) where a vacuum heating time is p (hour) and a radius of the disc or seat contacting the lower part of the sponge titanium block is r (mm) (with the proviso that r is 600 mm or longer).
 15. The method for producing sponge titanium by the Kroll method according to claim 7, wherein a total of a chlorine content and a magnesium content is 350 ppm by mass or lower and a filling density is 1.65 g/cm³ to 1.95 g/cm³ in the sponge titanium obtained by carrying out the crushing step.
 16. The method for producing sponge titanium by the Kroll method according to claim 7, wherein an average particle diameter of the sponge titanium obtained by carrying out the crushing step is 1.7 mm to 19.1 mm.
 17. The method for producing sponge titanium by the Kroll method according to claim 7, wherein a ratio of sponge titanium fine particles having a particle diameter of 0.84 mm or smaller in the sponge titanium obtained by carrying out the crushing step is 0.8% by mass or lower. 